Start by sitting in a chair with your shoulders relaxed.
Turn your head 45 degrees to one side and look down, tucking your chin into your chest (this position should make you look like your looking at your underarm).
Use the arm of the side your looking towards to gently pull your head further down.
You should feel a stretch on the back of your neck.
Hold this position as per physio instructions.
Repeat on the opposite side.
This is a sustained stretch, to avoid injury please avoid bouncing movements or stretching into pain.